I have not found an event being trigger that our plug-in can listen for related to a user sending a message to another user (just regular "IM"-style message, like in 1.9 and previous). The appropriate place for this would be in message_post_message. In fact, I could find not event triggered at all in this case, so no plug-in can hook into this. We would like to modify messages when they are sent with some metadata or redirect them to a user's "guardian" user if a teacher has specified such an option through the UI (one of our modifications as well). I think the events system is ideal for this, but it's currently not possible.

I filed this as a bug as opposed to a feature simply because the documentation of the Events API mentions a message_send event as something we can subscribe to, but I did not find any such event in the LMS code itself, and especially not one tied to the message_post_message() function (which is what I want specifically).